1. Field of the Invention
This application relates to improved systems and methods for the deliquification of liquid-containing substances by freeze-drying, particularly, dehydration of water-containing substances. By these systems and methods, a wide variety of substances can be deliquified, e.g., dried, more rapidly and economically than previously possible.
2. Description of the Prior Art
Freeze-drying is a method of dehydration of water-containing materials which yields a high quality, water free product. The high quality results from the nature of the process, which by definition involves the removal of water while the product is frozen. By remaining frozen during the dehydration, the product is largely protected from deleterious effects of heat, from the loss of volatile essences, and from adverse oxidation effects.
Removal of the water takes place by sublimation, i.e., vaporization of the solid without going through the liquid state, e.g., see U.S. Pat. No. 4,608,764. (Reference is made to water, but the liquid removed could be any that is capable of sublimation under the conditions involved.)
In conventional freeze-drying practice, the material is kept below freezing at very low pressure (essentially a vacuum) while providing the heat of vaporization and removing the vapor, e.g., see U.S. Pat. Nos. 2,471,035; 3,300,868; 3,362,835; 3,396,475; 3,909,957 and 4,016,657. Some systems have also been developed which operate at atmospheric total pressure, but very low partial pressures for the sublimation vapor, e.g., U.S. Pat. No. 3,313,032.
A key factor in such prior known systems is the relative slowness of drying. The simplest method used in practice is to freeze the material to be dried on trays, which are then loaded into a chamber equipped for the necessary vacuum, heating, and vapor removal. The vapor must penetrate through a relatively thick layer of frozen material, leading to typical drying cycles of 24 to 48 hours. Even in systems which work with thin layers or small particles, the usual cycles are still in the order of minutes to hours. Most such dryers operate in batch cycles since continuous freeze-dryers are typically much more complex and expensive. The equipment needed to achieve volume production generally becomes large and expensive.
In summation, existing freeze-drying processes and methods are slow, expensive, or both, resulting in their limited economic applicability despite well-known potential advantages of the freeze-drying concept. The present invention addresses these deficiencies of the prior art and provides improved systems and methods for the deliquification of liquid-containing substances by freeze-drying, particularly, dehydration of water-containing substances, that mitigate such prior art deficiencies.
While the terms "dehydration" and "drying", as used in this specification and the accompanying claims, concern principally the removal of water from aqueous materials, they are intended to encompass the deliquification of materials which contain liquids other than water, alone or in combination with water, e.g., organic solvents like alcohol, etc.